Eliminate system hum

A dedicated circuit is the best way to go. The Nation Elec.Code (NEC) has changed it's grounding requirements over the years. So it depends when you house was built or updated what kind of grounding you have. Also different States ,Counties and Cities have varying requirements. What you need is a dedicated ground and neutral to your System. The neutral is a current carring ground. It will transfer noise. Also if your electrical panel has a bonded ground the neutral and ground are connected at that point (carries noise). You need to "float" your ground to your music system. Talk to your electrician when installing a dedicated circuit about isolating you neutral and mechanical ground.

NEVER use a cheater plug!!!!

That is extremely dangerous and does not provide a way for parasitic currents to ground. Believe me you do not want to be the path to ground!!!

The answer is to ground all equipment in the system to a common potential i.e. a single circuit or circuits that use a common ground.

I had the same problem. Caused by cable tv coax cable. Try taking the coax cable out of the cable box. Then turn on your system and see if the hum is still there. But don't unplug the cable box from the wall. This will not do. You need to take out the coaxial cable that runs into your cable box, or whatever its first point of entry is.
I fixed this problem in my system with MONDIAL - MAGIC BOX. Bought it at musicdirect.com. No impact to picutre quality that I can detect. But the annoying hum noise is gone.
follow the link to check it out....
http://www.musicdirect.com/products/detail.asp?sku=AMONDBOX